Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder.
In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or states, such as a pressing state, a heating state, and a cooling state.
After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, preferably from 800 to 2000 bars and from 300° C. to 3000° C., preferably from 800° C. to 2000° C., respectively.
When the pressing of the articles is finished, the articles often need to be cooled before being removed, or unloaded, from the pressure vessel. In many kinds of metallurgical treatment, the cooling rate will affect the metallurgical properties. For example, thermal stress (or temperature stress) and grain growth should be minimized in order to obtain a high quality material. Thus, it is desired to cool the material homogeneously and, if possible, to control the cooling rate. Many presses known in the art suffer from slow cooling of the articles and efforts have therefore been made to reduce the cooling time of the articles. Notwithstanding the fact that a reduced cooling time is an important factor to take into account, high temperature uniformity during, for example, a steady-state state and a pressing state is also considerable importance. Thus, in addition to the capability of rapid cooling, a capability of achieving a high temperature uniformity during, for example, the steady-state state is desired.
Mechanical means for forcing a convective circulation may be applied to obtain an enhanced cooling rate. This is a way to achieve rapid cooling of treated articles although these are contained in a well insulated furnace chamber. Co-pending application PCT/EP2007/10997 discloses a hot isostatic pressing arrangement with these characteristics. Devices for forcing convection in high-pressure and/or high-temperature systems are however often subject to early wear or frequent machinery breakdown. This is in particular true of mechanical fans or ventilators with moving parts. Accordingly, such pressing arrangements may require relative frequent maintenance resulting in undesired production disruptions and stops.
Furthermore, the available space for loading articles in pressing arrangement is often limited and mechanical means such as fans or ventilators decrease this available space, a problem that is even more pronounced in smaller pressing arrangements.
The use of mechanical means also entails a relatively complex and expensive construction of the pressing arrangement.
Hence, there is still a need within the art of improved pressing arrangements capable of controlled, rapid cooling during a cooling state and of high temperature uniformity during steady-state and pressing.